VS Code on macOS resolves the shell environment at startup by running an
interactive login shell with stdout piped and no controlling terminal.
The order-200 auto-tmux block treated that probe as a normal interactive
shell and ran `exec tmux new-session`, which fails without a tty ("open
terminal failed: not a terminal") and exits non-zero. VS Code then reports
"Unable to resolve your shell environment: Unexpected exit code from
spawned shell (code 1)".
Gate the exec on a real terminal (-t 1) and skip it when
VSCODE_RESOLVING_ENVIRONMENT is set. Real terminals still land in tmux;
the integrated terminal was already exempt via TERM_PROGRAM.
Emma Thorpe